abstract class AbstractStringBuilder implements Appendable, CharSequence { /**
* The value is used for character storage.
*/
char[] value; /**
* The count is the number of characters used.
*/
int count;
}复制代码
/**
* Constructs a string builder with no characters in it and an
* initial capacity specified by the {@code capacity} argument.
*
* @param capacity the initial capacity.
* @throws NegativeArraySizeException if the {@code capacity}
* argument is less than {@code 0}.
*/
public StringBuilder(int capacity) { super(capacity);
} /**
* Constructs a string builder initialized to the contents of the
* specified string. The initial capacity of the string builder is
* {@code 16} plus the length of the string argument.
*
* @param str the initial contents of the buffer.
*/
public StringBuilder(String str) { super(str.length() + 16);
append(str);
} /**
* Constructs a string builder that contains the same characters
* as the specified {@code CharSequence}. The initial capacity of
* the string builder is {@code 16} plus the length of the
* {@code CharSequence} argument.
*
* @param seq the sequence to copy.
*/
public StringBuilder(CharSequence seq) { this(seq.length() + 16);
append(seq);
}复制代码
public String substring(int start, int end) { if (start < 0) throw new StringIndexOutOfBoundsException(start); if (end > count) throw new StringIndexOutOfBoundsException(end); if (start > end) throw new StringIndexOutOfBoundsException(end - start); return new String(value, start, end - start);
}复制代码
public String(char value[], int offset, int count) { if (offset < 0) { throw new StringIndexOutOfBoundsException(offset);
} if (count < 0) { throw new StringIndexOutOfBoundsException(count);
} // Note: offset or count might be near -1>>>1.
if (offset > value.length - count) { throw new StringIndexOutOfBoundsException(offset + count);
} this.value = Arrays.copyOfRange(value, offset, offset+count);
}复制代码
@Override
public synchronized String toString() { if (toStringCache == null) {
toStringCache = Arrays.copyOfRange(value, 0, count);
} return new String(toStringCache, true);
}复制代码
登入後複製
这个方法中首先判断当toStringCache 为null时会通过 Arrays.copyOfRange方法对其进行赋值，Arrays.copyOfRange方法上边已经分析过了，他会重新实例化一个char[]数组，并将原数组赋值到新数组中。这样做有什么影响呢？细细思考一下不难发现在不修改StringBuffer的前提下，多次调用StringBuffer的toString方法，生成的String对象都共用了同一个字符数组--toStringCache。这里是StringBuffer和StringBuilder的一点区别。至于StringBuffer中为什么这么做其实并没有很明确的原因，可以参考StackOverRun
《Why StringBuffer has a toStringCache while StringBuilder not?》中的一个回答:
